New synthetic approach to substituted isoindolo[2,1-a]quinoline carboxylic acids via intramolecular Diels–Alder reaction of 4-(N-furyl-2)-4-arylaminobutenes-1 with maleic anhydride

Acylation of substituted 4-(furyl-2)-4-arylaminobut-1-enes with maleic anhydride provided 2-allyl-6-carboxy-4-oxo-3-aza-10-oxatricyclo[5.2.1.01,5]dec-8-enes in high yield under mild reaction conditions. The Diels–Alder adducts are formed via an initial amide formation followed by a stereoselective intramolecular [4+2] exo-cycloaddition reaction. Treatment of the tricyclic compounds with phosphoric acid at high temperatures (70–120 °C) promoted cyclic ether opening, intramolecular cyclization and aromatization to give the corresponding tetracyclic compounds, 5,6,6a,11-tetrahydro-10-carboxyisoindolo[2,1-a]quinolines, in moderate yields. The influence of the acid and the reaction temperature on the cyclization reactions are also discussed.